Cells sense changes in their environment and respond, often in an all-or-none fashion, modulating motility, growth, developmental fate, or synaptic efficacy. Bacterial chemotaxis is a pre-eminent model system for studies of sensory transduction, where mechanisms can be understood in atomic detail. E. coli is a nanotechnological marvel, with cells only a micron in size propelled by several helical filaments, each driven at its base by a rotary motor 50 nm in diameter powered by a proton flux. When the filaments spin counterclockwise (CCW), a cell moves steadily forward - it "runs". When one or more filaments spin clockwise (CW), the cell changes course. A cell counts molecules of interest in its environment and extends runs deemed favorable. The counting is done by receptors that regulate the activity of a kinase that phosphorylates a response regulator that, when phosphorylated, diffuses across the cytoplasm and binds to the base of the flagellar motors, increasing the probability that they spin CW. Using fluorescence resonance energy transfer (FRET) between fluorescent fusion proteins in vivo, we will study receptor-receptor interactions responsible for high system gain, follow the diffusion of the response regulator within single cells, assess mechanisms for motor switching, and to try to learn whether there is feedback linking motors to receptors. We will test our understanding by computer modeling. New methods will be developed to probe motor function: to study proton-transfer in the high-speed limit, interactions of internal motor components, and behavior in an in vitro motor assay. Video analysis will be used to learn more about the motion of fluorescent flagellar filaments: how polymorphic transformations reorient the cell body, and how flagella on different cells interact to generate cooperative movement. While this effort is directly relevant to microbial virulence, it is meant as a study of fundamental biological processes: chemoreception, intracellular signaling, and conversion of chemiosmotic energy to mechanical work.